Conventionally, the so-called build-up circuit board is known. The build-up circuit board includes wiring layers and insulating layers that are alternately stacked, and via holes penetrating at least one insulating layer and electrically connecting at least two wiring layers. In the build-up circuit board having such a structure, the strength of the circuit board may be insufficient in a case in which the insulating layers do not include a reinforcing member. For this reason, the so-called prepreg having a reinforcing member impregnated with an insulating resin may be used as the outermost insulating layer, to strengthen the circuit board. For example, Japanese Laid-Open Patent Publication No. 2005-327780 (now Japanese Patent No. 4108643) proposes a circuit board having the prepreg insulating layer.
However, in a case in which the prepreg insulating layer is used and the outermost insulating layer is made thin, glass cloth or the like of the reinforcing member may make contact with the wiring layer embedded in the outermost insulating layer, to deteriorate the insulating reliability.
In addition, the prepreg has a high melting viscosity and the embeddability of the resin may be insufficient. For this reason, in a case in which the wiring layer embedded in the outermost insulating layer is a fine or micro wiring layer, the resin filling gaps in the wiring layer may be insufficient and generate voids or the like, to deteriorate the insulating reliability.